1. Field of the Invention
The present invention relates to an optical recording medium for recording and playing back an information signal by radiating a light beam thereon and a method of the production thereof.
2. Description of the Prior Art
FIG. 1 shows one example of a conventional single sided optical recording disc, and FIG. 2 is an enlarged cross sectional view of the center portion thereof. In FIGS. 1 and 2, pregrooves 12 corresponding to a tracking signal and prepits 13 corresponding to an address signal are transferred onto one surface of a disc substrate 11 of the optical recording medium, and a recording layer 14 made of a recording material for the heat mode is formed on the transferred surface where the pregrooves 12 and the prepits 13 are formed. The prepits 13 are formed on the pregrooves 12, and each of the information signals corresponding to the difference between the depths of the prepits 13 and the pregrooves 12 is read out.
In the case that an information signal is recorded in the recording layer 14 of the optical recording disc, the following process is performed. First of all, a laser beam for tracking is radiated from the side of the disc substrate 11 along the pregrooves 12 and the prepits 13, and the tracking information signal and the address information signal are detected by a receiver in a recording and playback device for detecting the light reflected on the recording layer 14. Next, a laser beam modulated by the predetermined information signal is radiated onto the predetermined track and the predetermined sector, so that thermal deformation such as melting, evaporation, sublimation, and contraction occurs in the recording layer 14, resulting in the formation of pits designating the information signal in the recording layer 14. On the other hand, in the case that the information signal recorded in the recording layer 14 of the optical recording disc is read out, the laser beam is radiated from the side of the disc substrate 11 along the pregrooves 12, and the tracking information signal and address information signal are detected by detecting the light reflected on the recording layer 14, resulting in the effect that the information signal recorded in the recording layer 14 can be detected.
Recently, a high polymeric material such as polymethylmethacrylate (referred to hereinafter as PMMA), polycarbonate (referred to hereinafter as PC), epoxy resin, etc. formed by injection molding is widely used as the disc substrate 11 provided for this kind of optical recording disc, because the inexpensive high polymeric material can be produced with a high productivity, this material is relatively light, not easily damaged, and the material can be transferred and dealt with easily.
Accordingly, since the high polymeric material has a relatively low heat resistance, the shapes of the pregrooves 12 and the prepits 13 transferred onto the disc substrate 11 may gradually change after the laser beam for playback is repeatedly radiated onto the recording layer 14, and problems develope, such as the contrast between the reflected light decreases and the carrier to noise level ratio (referred to hereinafter as the CN ratio) decreases.
Conventionally, in order to prevent the thermal deformation of the disc substrate 11 made of the high polymeric material during recording of the information signal, to prevent the projection of the disc substrate 11 around the portion where the recording pits are formed and the deformation of the pits formed by the projection of the disc substrate 11, and to record an information signal at a high CN ratio, an information recording medium comprising an adiabatic layer with a transparency between the disc substrate 11 and the recording layer 14 is publicly known (Japanese patent laid open No. 189356/1982), wherein the adiabatic layer is made of a transparent material having a high heat resistance such that the material cannot be deformed at a higher temperature than the temperature of the recording layer 14 when recording, and having a low heat conductivity and adiabatic effect. For example, the material is made of CuSe and As.sub.2 Se.sub.3, etc. including one of Se and S at a larger atomic percentage than 40 atomic percentage.
As the other examples, ultraviolet curing resin (Japanese patent laid open No. 86756/1978) and cellulose nitrate (Japanese patent laid open No. 55544/1982) are known. In the forming method of the layer made of the ultraviolet curing resin or the cellulose nitrate, either the rotating and coating method or the vacuum evaporation method is normally used. In this case, a thin layer made of Te is used as the material of the recording layer 14, and an organic compound such as ultraviolet curing resin, cellulose nitrate, etc. which can be coated by the rotating and coating method is used as an under layer arranged under the recording layer 14.
According to present research, the characteristic of the heat resistance of the under layer formed under the recording layer 14 remarkably influences the recording sensitivity of the recording layer 14, and it has been found that the thermal deformation of the recording layer 14 is facilitated by the thermal deformation of the under layer such that in the case of the disc substrate 11 the recording sensitivity of the recording layer 14 remarkably decreases when the thermal deformation does not occur at all. For example, in the case that the recording layer 14 is formed on the disc substrate 11 made of glass, the recording sensitivity remarkably decreases as compared that in the case when the recording layer 14 is formed on the disc substrate 11 made of PMMA. Accordingly, as the aforementioned known example, when an adiabatic layer wherein thermal deformation does not occur at all at a recording temperature is formed between the disc substrate 11 and the recording layer 14, thermal deformation of the disc substrate 11 can be prevented and the decrease of the CN ratio can be prevented. However, the recording sensitivity remarkably decreases, and there is a problem that the laser power must be increased.
For example, in the case when a ultraviolet curing resin is used as the under layer, there is a problem that the recording sensitivity decreases. On the other hand, in the case that cellulose nitrate is used as the under layer, the recording sensitivity does not decrease, however, it is necessary to perform the troublesome rotating and coating process with a relatively low productivity, and it is difficult to form the under layer made of cellulose nitrate at a uniform thickness. Moreover, recently the forming method of the recording layer made of Te widely used is the sputtering method, therefore, the rotating and coating method is not a desirable method.